CH SCIENCE

16 August 2013 Last updated at 04:37 ET

The prolific Kepler space telescope has had to give up its prime planet-hunting mission after engineers failed to find a fix for its hobbled pointing system.

Nasa's observatory lost the second of its four reaction wheels in May, meaning it can no long hold completely steady as it looks towards the stars.

Engineers have worked through a number of possible solutions but have failed to find one that will work.

Kepler has so far confirmed 135 planets beyond our Solar System.

But it still has more than 3,500 "candidates" in its database that have yet to be fully investigated, and the vast majority of these are expected to be confirmed as planets in due course.

"We really expect the most exciting discoveries are going to come in the next few years as we search through all this data," said Bill Borucki, the Kepler mission principal investigator.

The $600m (£395m) observatory was launched in 2009 to try to find Earth-sized worlds orbiting their host stars in the so-called habitable zone. This is the distance from the star where, given the right atmospheric conditions, temperatures might allow water to persist on a rocky surface in a liquid state. In essence, Kepler has been attempting to find planets that have the best chance of supporting life.

Its method of detection involved looking for the minute dips in light as planets passed in front of their stars. It is an extremely tricky measurement to make, with the total light changing just tiny fractions of a percent.

And it demanded Kepler be held absolutely still during the observations - something it needs a minimum of three spinning momentum wheels to achieve.

The spacecraft launched with four wheels, and experienced its first failure in the hardware set in July 2012. A second wheel went down earlier this year.

Officials liken the difficulty of controlling Kepler now to that of a shopper trying to push a supermarket trolley with a jammed wheel.

"The wheels are sufficiently damaged that they cannot sustain spacecraft pointing and control for any extended period of time," confirmed Charles Sobeck, Kepler's deputy project manager.

Kepler completed its prime mission in November 2012, so it has already worked beyond its minimum requirements. But Nasa still hopes to get some use out of the spacecraft, such as hunting perhaps for asteroids and comets. Ideas are being solicited from the scientific community.

More planet-hunting missions are due to come online in the coming years.

At the end of 2013, the European Space Agency will launch its Gaia observatory. Although its main goal is to map the positions of stars, it will do this so precisely that it should discover thousands of orbiting worlds in the process.